Smooth cast 300 and 320 both have the same cure times, demold times, pot life, tensile strength and hardness. 320 is designed to be pigmented, which I highly recommend, it helps you see the layers you are working with. 320 and 321 have the same physical properties, 321 just has a longer pot life and cure time, 7 minute pot life and 20 minute demold times instead of 3 minutes and 10 minutes. Other than pigmenting, I have seen that bondo has a tough time mechanically bonding with 300 sometimes, however I haven't had any issues with bondo and 320. You can find all the info on smooth on's website. I don't know where you guys got your facts, they seem pretty off to me.